Prune setup.cfg some more (#12059)

* Remove `trial` section from setup.cfg

This was added in the initial commit from 2014. I can't see that it does
anything. Maybe it's there so that you can run `trial` without any extra
args, but if I do that then I just get the `--help` message.

* Move flake8's config to its own file

.flake8

+# TODO: incorporate this into pyproject.toml if flake8 supports it in the future.
+# See https://github.com/PyCQA/flake8/issues/234
+[flake8]
+# see https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
+# for error codes. The ones we ignore are:
+#  W503: line break before binary operator
+#  W504: line break after binary operator
+#  E203: whitespace before ':' (which is contrary to pep8?)
+#  E731: do not assign a lambda expression, use a def
+#  E501: Line too long (black enforces this for us)
+ignore=W503,W504,E203,E731,E501

MANIFEST.in

@@ -45,6 +45,7 @@ include book.toml
 include pyproject.toml
 recursive-include changelog.d *
 
+include .flake8
 prune .circleci
 prune .github
 prune .ci

changelog.d/12052.misc

-Move `isort` configuration to `pyproject.toml`.
+Move configuration out of `setup.cfg`.

changelog.d/12059.misc

+Move configuration out of `setup.cfg`.
\ No newline at end of file

setup.cfg

-[trial]
-test_suite = tests
-
 [check-manifest]
 ignore =
     .git-blame-ignore-revs
@@ -10,12 +7,3 @@ ignore =
     pylint.cfg
     tox.ini
 
-[flake8]
-# see https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
-# for error codes. The ones we ignore are:
-#  W503: line break before binary operator
-#  W504: line break after binary operator
-#  E203: whitespace before ':' (which is contrary to pep8?)
-#  E731: do not assign a lambda expression, use a def
-#  E501: Line too long (black enforces this for us)
-ignore=W503,W504,E203,E731,E501